Ferritic stainless steel for exhaust system member having excellent corrosion resistance after heating

ABSTRACT

A ferritic stainless steel for exhaust system components excellent in corrosion resistance after heating includes: 0.015 mass % or less of C; 0.02 mass % or less of N; 0.03 mass % to 1.0 mass of Si; 1.0 mass of Mn; 0.04 mass of P; 0.01 mass of S; 10.5 mass % to 22.5 mass of Cr; 0.02 mass % to 0.5 mass of Sn; 0.003 mass % to 0.2 mass of Al; one or both of 0.03 mass % to 0.35 mass of Ti and 0.03 mass % to 0.6 mass of Nb; and a remnant comprising Fe and inevitable impurities, wherein a grain size number on a surface of the ferritic stainless steel is 6 to 2 to 15 nm of a layer containing Sn at a concentration twice or more of Sn content in the base material is formed on the ferritic stainless steel.

TECHNICAL FIELD

The present invention relates to a ferritic stainless steel excellent incorrosion resistance after heating and suitable for use in an exhaustsystem component for an automobile, a motor cycle, a commercial vehicleand a construction machine, an exhaust system component and amanufacturing method thereof. Specifically, the present inventionrelates to a ferritic stainless steel adapted to be heated to 573 to1073 K to be used with an oxide film being formed on a surface thereof.

BACKGROUND ART

Ferritic stainless steel is often used for exhaust system components ofan automobile, a motor cycle, a commercial vehicle, a constructionmachine and the like.

Especially, downstream exhaust system components (so-called “cold end”)are often made of SUH409L steel (a steel containing C and N fixed by Tiand 11% of Cr), SUS430LX steel (a steel containing C and N fixed by Tiand 17% of Cr), and SUS436J1L and SUS436L further added with Mo, interms of corrosion resistance, formability and weldability.

In view of recent growing concern over global environment issues,exhaust gas regulations and fuel consumption regulations are tightenedevery year. Accordingly, various measures have been studied andimplemented by automobile manufactures and automobile partsmanufactures. It is thus required to increase corrosion resistance orstrength of materials to reduce thickness and, consequently, weight ofthe materials. A demand for a longer guarantee period of the componentsalso necessitates the improvement in corrosion resistance.

Many of the exhaust system components are subjected to heating whenbeing welded for assembly to generate an oxide film (so-called “tempercolor”) at a welding heat-affected zone (HAZ). The oxide film issometimes generated during a travel of a vehicle depending on thelocation of the components. Thus, corrosion resistance of materials withthe oxide film being formed is practically important.

The corrosion resistance herein includes corrosion resistance againstcondensed water of exhaust gas on an interior surface and corrosionresistance against salt-induced corrosion on an exterior surface. Inmany cases, reduction in lifetime resulting in breakage due to localthickness reduction and generation of through pit(s) causing leakage ofexhaust gas are of problem. Accordingly, pitting resistance bears a highimportance in corrosion resistance. In addition, degradation inappearance due to generation of rust has recently been seen as aproblem.

Some solutions have been proposed for the above problems.

For instance, Patent Literature 1 discloses a stainless steel sheet withimproved crevice corrosion resistance, the stainless steel containing0.015% or less of C, 0.02% or less of N, 1.0% or less of Si, more than0.6% to 3.0% of Ni, 16.0 to 25.0% of Cr, optionally one or both of 3.0%or less of Mo and 2.0% or less of Cu as necessary, and one or more of2.0% or less of Mn, 0.5% or less of Ti, 0.5% or less of Nb, 0.5% or lessof Al and 0.01% or less of B, where a matrix with restricted amount of0.04% or less of P and 0.02% or less of S exhibits a ferritesingle-phase texture.

Patent Literature 2 discloses a ferritic stainless steel that isexcellent in crevice corrosion resistance, the ferritic stainless steelcontaining 0.001 to 0.02% of C, 0.001 to 0.02% of N, 0.01 to 0.3% of Si,0.05 to 1% of Mn, 0.04% or less of P, 0.15 to 2% of Ni, 11 to 22% of Cr,0.01 to 0.5% of Ti, and one or more of 0.5 to 3.0% of Mo, 0.02 to 0.6%of Nb and 0.1 to 1.5% of Cu in an amount satisfyingCr+3Mo+6(Ni+Nb+Cu)≧22. Both of Patent Literatures 1 and 2 relate to astainless steel containing Ni to provide improved crevice corrosionresistance, where corrosion growth speed is restrained to enhance thepitting resistance. However, nothing is disclosed on the corrosionresistance when an oxide film is formed by heating.

Patent Literature 3 discloses a ferritic stainless steel containing0.0010 to 0.30% of C, 0.0010 to 0.050% of N, 0.01 to 1.0% of Si, 0.01 to1.0% of Mn, 0.04% or less of P, 0.010% or less of S, 1.0% or less of Ni,10.0 to 30.0% of Cr, 0.010% or less of 0, 0.005 to 0.10% of one or bothof Sn and Sb, and, optionally, 0.0050 to 0.5% of Ti and/or 0.01 to 1.0%of Nb as necessary. The presence of one or both of Sn and Sb preventsgrain boundary segregation of P to restrain surface flaws caused due tointergranular corrosion during sulfuric acid pickling.

Patent Literature 4 discloses a manufacturing method of a steel platecontaining high-purity Cr that is excellent in pressing formability, thesteel plate containing 0.02% or less of C, 0.02% or less of N, 3 to 30%of Cr, and one or both of Ti and Nb in an amount satisfying(Ti+Nb)/(C+N)≧8, where a ferrite particle diameter of a cast product anda winding temperature during a hot rolling step are defined inpredetermined ranges. It is also disclosed that 0.5% or less of Sncontent is effective in order to restrain intergranular corrosion causedby Cr carbonitride.

Patent Literature 5 discloses a ferritic stainless steel that isexcellent in crevice corrosion resistance, the ferritic stainless steelcontaining 0.001 to 0.02% of C, 0.001 to 0.02% of N, 0.01 to 0.5% of Si,0.05 to 1% of Mn, 0.04% or less of P, 0.01% or less of S, 12 to 25% ofCr, one or both of 0.02 to 0.5% of Ti and 0.02 to 1% of Nb, and one orboth of 0.005 to 2% of Sn and 0.005 to 1% of Sb. Similarly to Ni inPatent Literatures 1 and 2, Patent Literature 5 relates to a stainlesssteel containing Sn and/or Sb to provide improved crevice corrosionresistance, where corrosion growth rate is inhibited to enhance thepitting resistance. However, nothing is disclosed in Patent Literatures3 to 5 on the corrosion resistance under the circumstances that an oxidefilm is formed by heating.

Patent Literature 6 discloses an alloy-saving ferritic stainless steelfor an automobile exhaust system component that is excellent incorrosion resistance after heating, the ferritic stainless steelcontaining 0.015% or less of C, 0.015% or less of N, 0.10 to 0.50% ofSi, 0.05 to 0.50% of Mn, 0.050% or less of P, 0.0100% or less of S, 10.5to 16.5% of Cr, one or both of 0.03 to 0.30% of Ti and 0.03 to 0.30% ofNb, and one or both of 0.03 to 0.50% of Sn and 0.03 to 0.50% of Sb in anamount satisfying Cr+Si+0.5Mn+10A1+15(Sn+Sb)≧13.

Patent Literature 7 discloses an Mo-saving ferritic stainless steel foran automobile exhaust system component that is excellent in corrosionresistance after heating, the ferritic stainless steel containing 0.015%or less of C, 0.015% or less of N, 0.01 to 0.50% of Si, 0.01 to 0.50% ofMn, 0.050% or less of P, 0.010% or less of S, 16.5 to 22.5% of Cr, 0.01to 0.100% of Al, one or both of 0.03 to 0.30% of Ti and 0.03 to 0.30% ofNb, and one or both of 0.03 to 1.00% of Sn and 0.05 to 1.00% of Sb.

Patent Literature 8 discloses a ferritic stainless steel for anautomobile exhaust system component, the ferritic stainless steelcontaining 0.015% or less of C, 0.015% or less of N, 0.01 to 0.50% ofSi, 0.01 to 0.50% of Mn, 0.050% or less of P, 0.010% or less of S, 0.5to 2.0% of Ni, 16.5 to 22.5% of Cr, 0.010 to 0.100% of Al, 0.01 to 0.50%of Sn, and one or both of 0.03 to 0.30% of Ti and 0.03 to 0.30% of Nb.All of Patent Literatures 6 to 8 disclose corrosion resistance under thecircumstances that an oxide film is formed by heating. However, thecomposition and formation conditions of the oxide film are not mentionedin Patent Literatures 6 to 8.

CITATION LIST Patent Literature(s)

Patent Literature 1 JP 2005-89828 A

Patent Literature 2 JP 2006-257544 A

Patent Literature 3 JP 11-92872 A

Patent Literature 4 JP 2002-38221 A

Patent Literature 5 JP 2008-190003 A

Patent Literature 6 JP 2010-31315 A

Patent Literature 7 JP 2010-116619 A

Patent Literature 8 JP 2011-190504 A

SUMMARY OF THE INVENTION Problem(s) to be Solved by the Invention

Thickness and weight reduction and increase in lifetime are demanded ofexhaust system components for an automobile, a motor cycle, a commercialvehicle, a construction machine and the like. Improvement in corrosionresistance is further required of downstream exhaust system components.An oxide film is locally formed on the components in practical use dueto heat applied during welding for assembly and travelling. The formedoxide film is inferior in corrosion resistance as compared with amaterial without the oxide film, and thus the pitting corrosion lifetimeand rust resistance are greatly influenced by the presence of the oxidefilm. Accordingly, an improvement in corrosion resistance with the oxidefilm being formed is effective for reducing the thickness, increasingthe lifetime and maintaining the good appearance of the component.

The invention has been achieved in view of the above problems. An objectof the invention is to provide a ferritic stainless steel excellent incorrosion resistance after heating and suitably usable as a material foran exhaust system component, an exhaust system component, and amanufacturing method of the ferritic stainless steel and the exhaustsystem component.

Means for Solving the Problem(s)

A summary of some of aspects of the invention capable of achieving theabove object is as follows.

-   (1) A ferritic stainless steel for exhaust system components    excellent in corrosion resistance after heating, the ferritic    stainless steel including: 0.015 mass % or less of C; 0.02 mass % or    less of N; 0.03 mass % to 1.0 mass % of Si; 1.0 mass % or less of    Mn; 0.04 mass % or less of P; 0.01 mass % or less of S; 10.5 mass %    to 22.5 mass % of Cr; 0.02 mass % to 0.5 mass % of Sn; 0.003 mass %    to 0.2 mass % of Al; one or both of 0.03 mass % to 0.35 mass % of Ti    and 0.03 mass % to 0.6 mass % of Nb; and a remnant including Fe and    inevitable impurities, in which a grain size number on a surface of    the ferritic stainless steel is 6 or more, and 2 to 15 nm of a layer    containing Sn at a concentration twice or more of a Sn concentration    in a base material is formed on the ferritic stainless steel when    the ferritic stainless steel is heated in the atmosphere under a    condition satisfying a formula (I),

exp(-−23000/T)×t≧4.3×10⁻¹⁵   (I)

where T represents a temperature (K) and t represents a time (s).

-   (2) A ferritic stainless steel for exhaust system components    excellent in corrosion resistance after heating, the ferritic    stainless steel including: 0.015 mass % or less of C; 0.02 mass % or    less of N; 0.03 mass % to 1.0 mass % of Si; 1.0 mass % or less of    Mn; 0.04 mass % or less of P; 0.01 mass % or less of S; 10.5 mass %    to 22.5 mass % of Cr; 0.02 mass % to 0.5 mass % of Sn; 0.003 mass %    to 0.2 mass % of Al; one or both of 0.03 mass % to 0.35 mass % of Ti    and 0.03 mass % to 0.6 mass % of Nb; and a remnant including Fe and    inevitable impurities, in which a grain size number on a surface of    the ferritic stainless steel is 6 or more, and 2 to 15 nm of a layer    containing Sn at a concentration twice or more of a Sn concentration    in a base material is formed on the ferritic stainless steel.-   (3) The ferritic stainless steel for exhaust system component    excellent in corrosion resistance after heating according to the    above aspects of the invention, further including at least one of    first group and a second group, the first group consisting of one or    more of 0.05 mass % to 1.5 mass % of Cu, 0.1 mass % to 1.2 mass % of    Ni, 0.03 mass % to 3 mass % of Mo, 0.03 mass % to 1 mass % of W,    0.05 mass % to 0.5 mass % of V and 0.01 mass % to 0.5 mass % of Sb,    the second group consisting of one or more of 0.03 mass % to 0.5    mass % of Zr, 0.02 mass % to 0.2 mass % of Co, 0.0002 mass % to    0.002 mass % of Ca, 0.0002 mass % to 0.002 mass % of Mg, 0.0002 mass    % to 0.005 mass % of B, 0.001 mass % to 0.01 mass % of REM, 0.0002    mass % to 0.01 mass % of Ga and 0.01 mass % to 0.5 mass % of Ta.-   (4) The ferritic stainless steel for exhaust system components    excellent in corrosion resistance after heating according to any one    of the above aspects of the invention, in which the Sn content is    0.02 mass % or more and less than 0.05 mass % and/or 0.07 mass % or    more and 0.3 mass % or less.-   (5) The ferritic stainless steel for exhaust system components    excellent in corrosion resistance after heating according to any one    of the above aspects of the invention, in which the Ni content is    0.1 mass % or more and less than 0.5 mass %.-   (6) An exhaust system component excellent in corrosion resistance    after heating, the exhaust system component being made from a    ferritic stainless steel, the ferritic stainless steel including:    0.015 mass % or less of C; 0.02 mass % or less of N; 0.03 mass % to    1.0 mass % of Si; 1.0 mass % or less of Mn; 0.04 mass % or less of    P; 0.01 mass % or less of S; 10.5 mass % to 22.5 mass % of Cr; 0.02    mass % to 0.5 mass % of Sn; 0.003 mass % to 0.2 mass % of Al; one or    both of 0.03 mass % to 0.35 mass % of Ti and 0.03 mass % to 0.6 mass    % of Nb; and a remnant including Fe and inevitable impurities, in    which a grain size number on a surface of the ferritic stainless    steel is 6 or more, and 2 to 15 nm of a layer containing Sn at a    concentration twice or more of a Sn concentration in a base material    is formed on the ferritic stainless steel.-   (7) The exhaust system component excellent in corrosion resistance    after heating and being made from the ferritic stainless steel    according to the above aspect of the invention, in which the    ferritic stainless steel further includes at least one of a first    group and a second group, the first group consisting of one or more    of 0.05 mass % to 1.5 mass % of Cu, 0.1 mass % to 1.2 mass % of Ni,    0.03 mass % to 3 mass % of Mo, 0.03 mass % to 1 mass % of W, 0.05    mass % to 0.5 mass % of V and 0.01 mass % to 0.5 mass % of Sb, the    second group consisting of one or more of 0.03 mass % to 0.5 mass %    of Zr, 0.02 mass % to 0.2 mass % of Co, 0.0002 mass % to 0.002 mass    % of Ca, 0.0002 mass % to 0.002 mass % of Mg, 0.0002 mass % to 0.005    mass % of B, 0.001 mass % to 0.01 mass % of REM, 0.0002 mass % to    0.01 mass % of Ga and 0.01 mass % to 0.5 mass % of Ta.-   (8) The exhaust system component excellent in corrosion resistance    after heating and being made from the ferritic stainless steel    according to any one of the above aspects of the invention, in which    the Sn content is 0.02 mass % or more and less than 0.05 mass %    and/or 0.07 mass % or more and 0.3 mass % or less.-   (9) The exhaust system component excellent in corrosion resistance    after heating and being made from the ferritic stainless steel    according to any one of the above aspects of the invention, in which    the Ni content is 0.1 mass % or more and less than 0.5 mass %.-   (10) A manufacturing method of the ferritic stainless steel for    exhaust system components excellent in corrosion resistance after    heating according to any one of the above aspects of the invention,    in which, when the ferritic stainless steel according to any one of    the above aspects of the invention is manufactured, a finish    annealing temperature in a cold rolling step is 1030 degrees C. or    less, and when the ferritic stainless steel is cooled from a    cold-rolled-sheet annealing temperature, a cooling rate in a    temperature range from 800 to 600 degrees C. is less than 20 degrees    C./s.-   (11) A manufacturing method of the ferritic stainless steel for    exhaust system components excellent in corrosion resistance after    heating according to any one of the above aspects of the invention,    in which when the ferritic stainless steel according to any one of    the above aspects of the invention is manufactured, a finish    annealing temperature in a cold rolling step is 1030 degrees C. or    less, and when the ferritic stainless steel is cooled from a    cold-rolled-sheet annealing temperature, a cooling rate in a    temperature range from 800 to 600 degrees C. is less than 5 degrees    C./s.-   (12) A manufacturing method of the exhaust system component    excellent in corrosion resistance after heating and being made from    the ferritic stainless steel according to any one of the above    aspects of the invention, in which, when the ferritic stainless    steel forming the exhaust system component according to any one of    the above aspects of the invention is manufactured, a finish    annealing temperature in a cold rolling step is 1030 degrees C. or    less, and when the ferritic stainless steel is cooled from a    cold-rolled-sheet annealing temperature, a cooling rate in a    temperature range from 800 to 600 degrees C. is less than 20 degrees    C./s.-   (13) A manufacturing method of the exhaust system component    excellent in corrosion resistance after heating and being made from    the ferritic stainless steel according to any one of the above    aspects of the invention, in which when the ferritic stainless steel    forming the exhaust system component according to any one of the    above aspects of the invention is manufactured, a finish annealing    temperature in a cold rolling step is 1030 degrees C. or less, and    when the ferritic stainless steel is cooled from a cold-rolled-sheet    annealing temperature, a cooling rate in a temperature range from    800 to 600 degrees C. is less than 5 degrees C./s.

The ferritic stainless steel of the above aspects of the invention issuitable for a material of exhaust system components of an automobile, amotor cycle, a commercial vehicle, a construction machine and the like.Since the ferritic stainless steel of the above aspects of the inventionimproves the corrosion resistance of a portion including a weldedportion that is subjected to heating in use, the ferritic stainlesssteel contributes to an increase in lifetime of the exhaust systemcomponent and thickness and weight reduction of the exhaust systemcomponent.

BRIEF DESCRIPTION OF DRAWING(S)

FIG. 1 shows an influence of Sn content exerted on a maximum pit depth.

DESCRIPTION OF EMBODIMENT(S)

Exemplary embodiment(s) of the invention will be described below indetail.

In studying corrosion resistance after heating, the inventors of theinvention focused on an oxide film formed by the heating and conducteddetailed studies. This is because the inventors suspected that thedeterioration in corrosion resistance due to heating was primarilydependent on a formation status of the oxide film.

When a ferritic stainless steel is heated at 573 to 1073 K in theatmosphere, an oxide film having Fe-rich external layer and Cr-richinternal layer is formed on the surface of the ferritic stainless steel.The oxide film is inferior to a passivation film of unheated stainlesssteel in terms of performance for shielding a material from a corrosiveenvironment. Accordingly, with an identical chemical composition of thematerial, the heated material is inferior in corrosion resistance. Thus,it is believed that an improvement in the formation status of the oxidefilm would lead to an improvement in corrosion resistance after heating.However, since the ferritic stainless steel is mostly formed of Fe andCr, it is inevitable that the oxide film is primarily formed of thesetwo elements. Accordingly, a use of a third element other than Fe and Cris attempted.

When steel is heated at a temperature ranging from 573 to 1073 Kapproximately for 24 hours at the maximum, though an oxide film madeprimarily of the primary elements (i.e. Fe and Cr) having a thickness ofapproximately 20 nm to sub-microns is formed, it is difficult toconcentrate minute amounts of metal element(s) added in the steel in theentirety of the oxide film. Accordingly, it was attempted to concentratethe element(s) effective for improving corrosion resistance at and nearthe border between the oxide film and a base material. If the element(s)is less likely to be oxidized than Fe and Cr, the element(s) can beconcentrated in a metal form in an environment in which Fe and Cr areoxidized (e.g. in the atmosphere). In view of the above, Cu, Ni and Snare studied as the above elements in terms of corrosion resistance.

When oxide films formed after heating in the atmosphere on ferriticstainless steels each added with Cu alone, Ni alone and Sn alone werecompared, it was found that the element most likely to concentrate atthe border between the oxide film and the base material among the threeelements was Sn. As a result of chemical status analysis using an X-rayphotoelectric spectroscopy (referred to as XPS hereinafter), it wasfound that Sn was concentrated in a metal form.

Then, ferritic stainless steel sheets containing0.004C-0.008N-0.1Si-0.1Mn-16.5Cr-0.2Nb-0.1Ti-0.03A1 system (the numbersrepresenting contents of individual elements (mass %)) as a basecomponent and Sn content ranging from 0 to 0.5 mass % were prepared assamples, and each of the samples was subjected to a heat treatment inthe atmosphere at 673 K for 24 hours and, subsequently, was subjected totwo cyclic corrosion tests. It should be noted that, when the steelsheets were cooled from a finish annealing temperature during theproduction of the steel sheet, the cooling rate of the steel sheets was15 degrees C./s in a temperature range from 800 to 600 degrees C. Grainsize on a Z-surface of the steel plates was 6.5.

In the first one of the cyclic corrosion tests, which is intended toassess pitting corrosion resistance, one cycle of: spraying 5% NaClsolution at 35 degrees C. for two hours; drying at 60 degrees C. forfour hours; and wetting at 50 degrees for two hours in accordance withJASO M609-91 was repeated for 120 times (i.e. 120 cycles). Aftercompletion of the test, corrosion product was removed using di-ammoniumhydrogen citrate aqueous solution. Subsequently, a maximum pit depth wasmeasured using a microscope focal depth method. In the second one of thecyclic corrosion tests, which is intended to assess rust resistance, onecycle of: spraying ten-fold diluted artificial seawater at 35 degrees C.for four hours; drying at 60 degrees C. for two hours; and wetting at 50degrees C. for two hours was repeated for three times (i.e. threecycles). After completion of the test, rust generation level was gradedusing a rating number (abbreviated as RN hereinafter) according to JISG0595. It should be noted that a larger number of RN represents a moreexcellent rust resistance.

FIG. 1 shows an influence of the Sn content on the maximum pit depthmeasured in the first test. As shown in FIG. 1, it is understood thatthe presence of 0.02 mass % or more of Sn clearly reduces the maximumpit depth and the maximum pit depth is reduced in accordance with anincrease in the Sn content. On the other hand, though RN rated in thesecond test was 5 when the Sn content was 0.001%, RN was 6 or more when0.02 mass % or more of Sn was contained (i.e. the rust generation levelwas improved). The presence of rust is easily observable and degradationof appearance is clearly recognizable when RN is 5. Accordingly, thesteel is judged inferior in quality when RN is 5 or less whereas thesteel is judged excellent when RN is 6 or more. As described above, itis demonstrated that the presence of 0.02 mass % or more of Sn canimprove the rust resistance in addition to the pitting corrosionresistance.

A sample containing 0.021 mass % of Sn was subjected to the same heattreatment as that in the above corrosion test and was examined using theXPS. It was found that an approximately 40-nm-thick oxide film having anFe-rich external layer and a Cr-rich internal layer was formed on asurface of the sample and 0.02 to 0.04 at % in cation fraction of Sn waspresent in a region of approximately 2 nm at and near the border betweenthe oxide film and the base material. The Sn content at and near theborder between the oxide film and the base material increases inaccordance with an increase in the Sn content in the sample. When 0.5mass % of Sn was contained in the sample, 0.47 to 0.7 at % in cationfraction of Sn was detected over a region of approximately 10 nm. Sincethe Sn amount in the base material containing 0.5 mass % of Sn isapproximately 0.22 at %, it is clear that Sn is concentrated at and nearthe border between the oxide film and the base material. In theinvention, a layer present at and near the border between the oxide filmand the base material and containing Sn at a larger concentration thanthe Sn content in the base material will be referred to as anSn-concentrated layer hereinafter. It is found that, when the thicknessof the Sn-concentrated layer is 2 nm or more and the Sn concentration inthe Sn-concentrated layer is twice or more of the Sn content in the basematerial, the effect for improving the corrosion resistance of theinvention can be exhibited.

The Sn content and thickness of the Sn-concentrated layer increase inaccordance with an increase in heating temperature and heating time.However, excessive heating results in an uneven growth of the oxide filmand, consequently, uneven thickness of the Sn-concentrated layer, andalso results in saturation of corrosion resistance improving effect.With the maximum heating temperature and time (i.e. at 1073 K for 24hours), the thickness of the Sn-concentrated layer was approximately 15nm.

Though the reason why the concentration of Sn at and near the borderbetween the oxide film and the base material effectively reduces the pitdepth in the cyclic corrosion test and improves the rust resistance isnot fully understood, it is speculated that this is because dissolvedand ionized Sn serves as an inhibitor (i.e. a corrosion inhibitor). Theprogress speed of the pitting corrosion is thus reduced to decrease thepit depth and growth of lately generated small pit is stopped, therebyimproving the rust resistance. It is believed that, since the Sn ispresent in a metal form at and near the border between the oxide filmand the base material at a higher concentration than in the basematerial, the corrosion of the base material can be more effectivelyrestrained.

In order to concentrate Sn at and near the border between the oxide filmand the base material, it is preferable that heating is performed in theatmosphere so that the following formula (I) is satisfied.

exp(−23000/T)×t≧4.3×10⁻¹⁵   (I)

T: temperature (K), t: time (s)

A preferable value for the right side of the formula (I) is 8.6×10⁻¹⁵.On the other hand, since excessive heating results in saturation of theconcentration of Sn, the upper limit of the value represented by theleft side of the formula (I) is preferably 4.5×10⁻⁹.

Further, though Fe and Cr are oxidized by heating to promote theconcentration of Sn at and near the border between the oxide film andthe base material, it is necessary that a grain size number on thesurface is 6 or more in order to reach the Sn concentration required inthe invention. Grain boundary diffusion dominantly occurs in thetemperature range of 573 to 1073 K. Accordingly, with a small grainsize, the diffusion of Sn is promoted to progress the concentration ofSn. Preferably, the grain size number is 6.5 or more and, morepreferably, 7 or more. In addition, it is effective to form a processedlayer on the surface by polishing and the like in order to concentrateSn.

The effects of each of alloy elements of the invention and the reasonfor the specified content thereof will be detailed below. In thefollowing, % refers to mass % except otherwise defined.

C: 0.015% or Less

Since C decreases intergranular corrosion resistance and formability,the content of C should be kept at a low level. Accordingly, the upperlimit of the C content is set at 0.015%, preferably at 0.012%. However,when the C content is excessively low, a necessary strength cannot beobtained and refining cost increases. Accordingly, the lower limit ofthe C content is preferably set at 0.002%, more preferably at 0.003%.

N: 0.02% or Less

Though N is an element effective for improving pitting resistance, sinceN decreases intergranular corrosion resistance and formability, thecontent of N should be kept at a low level. Accordingly, the upper limitof the N content is set at 0.02%, preferably at 0.018%. However, whenthe N content is excessively low, necessary strength cannot be obtainedand refining cost increases. Accordingly, the lower limit of the Ncontent is preferably set at 0.002%, more preferably at 0.003%.

Si: 0.03% or More and 1.0% or Less

Since Si is an element effective for improvement in oxidation resistanceand adapted to improve the corrosion resistance after heating, it isnecessary that Si content is 0.03% or more. The lower limit of Sicontent is preferably 0.05%, more preferably 0.1%, further preferably0.2%. However, since the addition of excessive amount of Si results indecrease in formability, the upper limit of Si content is set at 1.0%.The upper limit of Si content is preferably 0.8%, more preferably 0.6%,further preferably 0.5%.

Mn: 1.0% or Less

Since Mn deteriorates corrosion resistance, the content of Mn has to belimited. Accordingly, the upper limit of the Mn content is set at 1.0%,preferably at 0.5%. However, extremely lowering the Mn content resultsin an increase in the production cost. Accordingly, the lower limit ofthe Mn content is preferably set at 0.03%, more preferably at 0.05%.

P: 0.04% or Less

Since P deteriorates formability and weldability, the content of P hasto be limited. Accordingly, the upper limit of the P content is set at0.04%. However, extremely lowering the P content results in an increasein the production cost. Accordingly, the upper limit of the P content ispreferably set at 0.02%.

S: 0.01% or Less

Since S deteriorates corrosion resistance, the content of S has to belimited. Accordingly, the upper limit of the S content is set at 0.01%,preferably at 0.005%, more preferably at 0.002%.

Cr: 10.5% or More and 22.5% or Less

Since Cr is a basic element for ensuring the corrosion resistance, thelower limit of Cr content has to be set at 10.5%. Preferably, the Crcontent is 11.0% or more, more preferably 12.5% or more, furtherpreferably 14.0% or more. On the other hand, though the corrosionresistance can be improved in accordance with increase in the Crcontent, excessive addition of Cr leads to deterioration in formabilityand productivity. Accordingly, the Cr content is 22.5% or less,preferably 20.5% or less, further preferably 19.5% or less, further morepreferably 18.0% or less.

Sn: 0.02% or More and 0.5% or Less

Sn is an element extremely useful for improving the corrosion resistanceafter heating, and is the most important in the invention. Accordingly,the lower limit of the Sn content is set at 0.02%, preferably at 0.05%,more preferably at 0.07% and further preferably at 0.1%. On the otherhand, though the corrosion resistance after heating can be improved inaccordance with an increase in the Sn content, excessive addition of Snleads to deterioration in formability and productivity. Accordingly, theSn content is 0.5% or less, preferably 0.4% or less, further preferably0.3% or less, further more preferably 0.25% or less. In addition, it ispreferable to adjust the Sn content depending on the required level ofthe corrosion resistance after heating. Specifically, when the requiredlevel of the corrosion resistance after heating is low, the Cr contentis suitably defined to be 0.02% or more and less than 0.05%. When anormal level of the corrosion resistance after heating is required, theCr content is suitably defined to be 0.07% or more and 0.3% or less.When the required level of the corrosion resistance after heating ishigh, the Cr content is suitably defined to be 0.3% or more and 0.5% orless. It is more preferable that the Cr content is 0.1% or less when thenormal level of the corrosion resistance after heating is required.

Al: 0.003% or More and 0.2% or Less

Al is effective as a deoxidizing element and it is necessary that 0.003%or more of Al is contained. Al content is preferably 0.005% or more,more preferably 0.01%. However, since the addition of excessive amountof Al results in deterioration in toughness and productivity, the upperlimit of Al content is set at 0.2%. The upper limit of Al content ispreferably 0.15%, more preferably 0.1%.

The stainless steel of the exemplary embodiment contains one or both ofTi and Nb in the following amount.

Ti: 0.03% or More and 0.35% or Less

Ti is an element that is fixed with C and N to form a Ti carbonitride toinhibit intergranular corrosion. Further, Ti is also fixed with S toform a Ti sulfide or Ti carbon-sulfide to improve the corrosionresistance. Accordingly, the lower limit of the Ti content is set at0.03%, preferably at 0.05%, more preferably at 0.07%. However, since theaddition of excessive amount of Ti results in an adverse effect in termsof formability and productivity, the upper limit of Ti content is set at0.35%. The upper limit of the Ti content is preferably 0.32%, morepreferably 0.28%. It should be noted that the Ti content should be4(C+N)+3S or more.

Nb: 0.03% or More and 0.6% or Less

Similarly to Ti, Nb is an element that is fixed with C and N to form anNb carbonitride to inhibit the intergranular corrosion. In addition, Nbacts to improve high-temperature strength. Accordingly, the lower limitof the Nb content is set at 0.03%, preferably at 0.1%, more preferablyat 0.2%. However, since the addition of excessive amount of Nb resultsin an adverse effect in terms of formability, the upper limit of Nbcontent is set at 0.6%. The upper limit of Nb content is preferably0.55%, more preferably 0.5%.

The stainless steel of the exemplary embodiment may optionally furthercontain, in mass %, one or more of 0.05 to 1.5% of Cu, 0.1 to 1.2% ofNi, 0.03 to 3% of Mo, 0.03 to 1% of W, 0.05 to 0.5% of V, and 0.01 to0.5% of Sb.

Cu: 0.05% or More and 1.5% or Less

Cu is an element that enhances corrosion resistance and strength.Accordingly, 0.05% or more of Cu may be added as necessary. The Cucontent is preferably 0.2% or more, more preferably 0.3% or more.However, since the addition of excessive amount of Cu results indecrease in formability, the upper limit of Cu content is preferably setat 1.5% or less. The Cu content is more preferably 1.0% or less andfurther preferably 0.8% or less.

Ni: 0.1% or More and 1.2% or Less

Ni is an element that enhances corrosion resistance. Accordingly, 0.1%or more of Ni may be added as necessary. Ni content is preferably 0.2%or more, more preferably 0.3% or more. However, excessive addition ofNi, which is expensive, results in deterioration in formability and inan increase in the production cost. Accordingly, the Ni content ispreferably 1.2% or less, more preferably 0.9% or less and furtherpreferably less than 0.5%.

Mo: 0.03% or More and 3% or Less

Mo is an element that enhances corrosion resistance and strength.Accordingly, 0.03% or more of Mo may be added as necessary. Preferably,the Mo content is 0.1% or more, more preferably 0.3% or more, furtherpreferably 0.7% or more. However, excessive addition of Mo results indeterioration in formability and, since Mo is expensive, increase in theproduction cost. Accordingly, the Mo content is preferably 3% or less,more preferably 2.2% or less and further preferably 1.8% or less.

W: 0.03% or More and 1% or Less

W is an element that enhances corrosion resistance. Accordingly, 0.03%or more of W may be added as necessary. The W content is preferably 0.2%or more, more preferably 0.5% or more. However, excessive addition of W,which is expensive, results in deterioration in formability and in anincrease in the production cost. Accordingly, the W content ispreferably 1% or less, more preferably 0.8% or less.

V: 0.05% or More and 0.5% or Less

V is an element that enhances corrosion resistance. Accordingly, 0.05%or more of V may be added as necessary. The V content is furtherpreferably 0.1% or more. However, excessive addition of V, which isexpensive, results in deterioration in formability and in an increase inthe production cost. Accordingly, the V content is preferably 0.5% orless, more preferably 0.3% or less.

Sb: 0.01% or More and 0.5% or Less

Sb is an element that enhances corrosion resistance. Accordingly, 0.01%or more of Sb may be added as necessary. The Sb content is preferably0.03% or more, more preferably 0.05% or more. However, excessiveaddition of Sb results in deterioration in formability and productivity.Accordingly, the Sb content is preferably 0.5% or less, more preferably0.3% or less.

The stainless steel of the exemplary embodiment may optionally furthercontain, in mass %, one or more of 0.03 to 0.5% of Zr, 0.02 to 0.2% ofCo, 0.0002 to 0.002% of Ca, 0.0002 to 0.002% of Mg, 0.0002 to 0.005% ofB, 0.001 to 0.01% of REM, 0.0002 to 0.01% of Ga, and 0.01 to 0.5% of Ta.

Zr: 0.03% or More and 0.5% or Less

Zr is an element that enhances corrosion resistance, especiallyintergranular corrosion resistance. Accordingly, 0.03% or more of Zr maybe added as necessary. The Zr content is preferably 0.05% or more, morepreferably 0.1% or more. However, excessive addition of Zr results indeterioration in formability and, since Zr is expensive, increase in theproduction cost. Accordingly, the Zr content is preferably 0.5% or less,more preferably 0.3% or less.

Co: 0.02% or More and 0.2% or Less

Co is an element that enhances secondary formability and toughness.Accordingly, 0.02% or more of Co may be added as necessary. The Cocontent is preferably 0.05% or more, more preferably 0.08% or more.However, excessive addition of Co results in an increase in theproduction cost. Accordingly, the Co content is preferably 0.2% or less,more preferably 0.18% or less.

Ca: 0.0002% or More and 0.002% or Less

Ca is an element that has deoxidization effect and the like and thus isuseful in a refining process. Accordingly, 0.0002% or more of Ca may beadded as necessary. The Ca content is more preferably 0.0004% or more.However, since Ca forms Ca sulfide to deteriorate the corrosionresistance, the Ca content is preferably 0.002% or less and morepreferably 0.0015% or less.

Mg: 0.0002% or More and 0.002% or Less

Mg is an element that has deoxidization effect and the like and thus isuseful in a refining process. In addition, Mg miniaturizes the textureto improve formability and toughness. Accordingly, 0.0002% or more of Mgmay be contained, and more preferably 0.0005% or more of Mg may becontained as necessary. However, since excessive addition of Mgdeteriorates the corrosion resistance, the Mg content is preferably0.002% or less and more preferably 0.0015% or less.

B: 0.0002% or More and 0.005% or Less

B is an element that enhances formability, especially secondaryformability. Accordingly, 0.0002% or more of B may be added asnecessary. The B content is more preferably 0.0003% or more. However,since excessive addition of B deteriorates intergranular corrosionresistance, the B content is preferably 0.005% or less and morepreferably 0.002% or less.

REM: 0.001% or More and 0.01% or Less

REM represents a group of elements including La, Y, Ce, Pr, Nd and thelike belonging to atomic numbers of 57-71. REM is a group of elementsthat have deoxidization effect and the like and thus is useful in arefining process. Accordingly, 0.001% or more of REM may be added asnecessary. However, excessive addition of REM results in an increase inthe production cost. Accordingly, the REM content is preferably 0.01% orless.

Ga: 0.0002% or More and 0.01% o r Less

Ga is an element that forms a stable sulfide to improve corrosionresistance and hydrogen embrittlement resistance. Accordingly, 0.0002%or more of Ga may be added as necessary. However, excessive addition ofGa results in an increase in the production cost. Accordingly, the Gacontent is preferably 0.01% or less.

Ta: 0.01% or More and 0.5% or Less

Ta is an element that enhances the corrosion resistance. Accordingly,0.01% or more of Ta may be added as necessary. The Ta content ispreferably 0.05% or more, more preferably 0.1% or more. However,excessive addition of Ta results in decrease in toughness and increasein the production cost. Accordingly, the Ta content is preferably 0.5%or less, more preferably 0.4% or less.

The stainless steel of the exemplary embodiment is basicallymanufactured according to a method typically employed in order tomanufacture ferritic stainless steel. For instance, molten steel havingthe above chemical composition may be produced in a converter furnace oran electric furnace, refined in an AOD furnace or a VOD furnace, andformed into a steel piece through a continuous casting process oringot-making process. The steel piece is then sequentially subjected tohot rolling, hot-rolled sheet annealing, pickling, cold rolling-finishannealing, and pickling. The hot-rolled sheet annealing may be omittedand/or the sequence of cold rolling, finish annealing and pickling maybe repeated as necessary. A use of a small-diameter roller having adiameter of 150 mm or less in the cold rolling step is effective forconcentrating Sn at and near the border between the oxide film and thebase material. Further, in order to promote recrystallization, thefinish annealing temperature is preferably 800 degrees C. or more and,in order to restrain the grains from being coarsened, the finishannealing temperature is preferably 1030 degrees C. or less. Further, inorder to enhance grain boundary segregation of Sn and to promote theconcentration of Sn at and near the border between the oxide film andthe base material, it is preferable that the cooling rate in atemperature range from 800 to 600 degrees C. during cooling from thefinish annealing temperature is less than 20 degrees C./s on average.More preferably, the cooling rate is less than 15 degrees C./s onaverage, more preferably less than 5 degrees C./s on average.

To produce a ferritic stainless steel sheet containing the abovecomponents of the exemplary embodiment, the finish annealing temperatureof the cold rolling is set at an appropriate temperature of 1030 degreesC. or less and the cooling rate in a temperature range from 800 to 600degrees C. during cooling from the finish annealing temperature is lessthan 20 degrees C./s on average, so that the grain size number on thesurface of the steel becomes 6 or more. Accordingly, when the ferriticstainless steel sheet is heated in the atmosphere under the conditionsatisfying the formula (I), 2 to 15 nm of the layer containing Sn at aconcentration twice or more of the Sn content in the base material canbe formed.

Further, when a ferritic stainless steel sheet containing the abovecomponents of the exemplary embodiment is to be produced, the finishannealing temperature of the cold rolling is set at an appropriatetemperature of 1030 degrees C. or less, the cooling rate in atemperature range from 800 to 600 degrees C. during cooling from thefinish annealing temperature is less than 20 degrees C./s on average andthe steel sheet is heated in the atmosphere under the conditionsatisfying the formula (I), so that a ferritic stainless steel sheetwhose grain size number on the surface is 6 or more and having 2 to 15nm of a layer containing Sn at a concentration twice or more of the Sncontent in the base material can be manufactured.

The heating in the atmosphere under the condition satisfying the formula(I) corresponds to the heating applied on the exhaust system componentwhen a vehicle travels. The heating in the atmosphere under thecondition satisfying the formula (I) may be applied on a steel sheetbefore being assembled into the exhaust system component.

The exhaust system component excellent in corrosion resistance afterheating according to the exemplary embodiment is manufactured using thesteel plate as a material according to a typical manufacturing method ofa stainless steel pipe for exhaust system components such as electricresistance welding, TIG welding and laser welding.

EXAMPLES

The invention will be described in more details with reference toExamples.

1 mm-thick cold-rolled steel sheets were prepared by: melting stainlesssteel of the compositions shown in Table 1-1 in a 180 kg vacuum meltingfurnace; casting the stainless steel into steel ingots of 45 kg; andsubjecting the steel ingots to a process including hot rolling,hot-rolled sheet annealing, shot blasting, cold rolling, and finishannealing. Each of the hot-rolled sheets was prepared by rolling each ofthe ingots of 50 mm thickness to a thickness of 5 mm at a heatingtemperature of 1200 degrees C. and subsequently air-cooling thehot-rolled sheet. The hot-rolled sheet annealing was applied byair-cooling for one minute in a temperature range from 850 to 1050degrees C. Subsequently, scales were removed by the shot blasting. Then,each of the steel sheets was cold-rolled to obtain a 1-mm-thick steelsheet and was subjected to the finish annealing in which the steelsheets were held for one minute under the temperatures shown in Table1-2. Thereafter, the steel plates were cooled under the conditions shownin Table 1-2.

A specimen of 70 mm in width and 150 mm in length was cut out from eachof the cold-rolled steel sheets. A test surface of the specimen waswet-polished up to #600 using Emery paper. Subsequently, the specimenwas subjected to a heat treatment at 673 K in the atmosphere for 24hours. The value represented by the left side of the formula (I) at thistime is 1.2×10⁻¹⁰. For comparison sake, Comparative Example 5 in Table1-2 (steel 7) was subjected to a heat treatment at 523 K in theatmosphere for 15 minutes instead of the heat treatment at 673 K for 24hours. The value represented by the left side of the formula (I) at thistime is 7.1×10⁻¹⁷.

The distribution of the Sn content at and near the surface of the steelsheet after the heat treatment was measured using an XPS. When thespecimen used for the aforementioned cyclic corrosion test was subjectedto the heat treatment, the sample for the surface analysis wassimultaneously subjected to the heat treatment. The XPS was manufacturedby ULVAC-PHI, Inc. having an X-ray source of mono-Al K_(α)ray, whereelemental analysis in the depth direction was performed using Ar-ionsputtering. The sputtering rate was 1.5 nm/min in terms of SiO₂. Thethickness of the Sn-concentrated layer present at the border regionbetween the oxide film and the base material was measured (shown inTable 1-2). The thickness of the Sn-concentrated layer represents athickness of the region in which detected Sn concentration was higherthan the Sn content in the base material. The lowest Sn concentration inthe Sn-concentrated layer is shown in Table 1-2 in atom %. A valueobtained by dividing the Sn concentration in the Sn-concentrated layerby the Sn content in the base material is shown in Table 1-2 as“Concentration Degree.”

The corrosion resistance was evaluated using the two types of cycliccorrosion tests. In the first one of the cyclic corrosion tests, onecycle of: spraying 5% NaCl solution at 35 degrees C. for two hours;drying at 60 degrees C. for four hours; and wetting at 50 degrees fortwo hours in accordance with JASO M609-91 was repeated for 120 times(i.e. 120 cycles). After completion of the cyclic corrosion test,corrosion product was removed using di-ammonium hydrogen citrate aqueoussolution. Subsequently, a maximum pit depth was measured using amicroscope focal depth method. In the second one of the cyclic corrosiontests, one cycle of: spraying ten-fold diluted artificial seawater at 35degrees C. for four hours; drying at 60 degrees C. for two hours; andwetting at 50 degrees for two hours was repeated for three times (i.e.three cycles). After completion of the test, a rust generation level wasgraded using the rating number according to JIS G0595.

A specimen of 20 mm in width and 20 mm in length was cut out from thesame cold-rolled steel sheet. A surface of the specimen wasmirror-polished and subsequently etched to expose microstructure. Agrain size on a Z-surface (a surface parallel to the surface) wasmeasured in accordance with JIS G0551.

TABLE 1-1 Chemical Composition (mass %) C N Si Mn P S Cr Sn Ti Nb AlOthers Inventive Ex. 1 Steel 1 0.004 0.018 0.49 0.26 0.025 0.002 17.890.13 0.34 — 0.004 — 2 Steel 2 0.012 0.017 0.21 0.06 0.021 0.001 14.110.24 — 0.49 0.031 — 3 Steel 3 0.003 0.004 0.11 0.11 0.029 0.003 19.430.07 0.27 — 0.078 — 4 Steel 4 0.007 0.014 0.78 0.04 0.031 0.001 10.610.49 0.06 0.31 0.003 — 5 Steel 5 0.006 0.013 0.60 0.09 0.032 0.001 11.120.46 0.21 — 0.010 — 6 Steel 6 0.005 0.011 0.51 0.10 0.026 0.004 12.780.39 0.22 0.04 0.021 — 7 Steel 7 0.004 0.008 0.20 0.09 0.025 0.002 13.870.03 0.13 0.12 0.036 — 8 Steel 8 0.004 0.005 0.06 0.94 0.022 0.001 20.450.05 0.09 0.19 0.085 — 9 Steel 9 0.004 0.005 0.05 0.48 0.023 0.001 22.430.03 0.11 0.21 0.092 — 10 Steel 10 0.010 0.015 0.26 0.10 0.023 0.00113.97 0.22 — 0.48 0.025 0.32Cu 11 Steel 11 0.009 0.014 0.31 0.14 0.0250.001 14.52 0.19 0.31 — 0.029 0.38Ni 12 Steel 12 0.007 0.012 0.19 0.150.028 0.001 14.33 0.15 — 0.58 0.019 0.74Mo, 0.27Ni 13 Steel 13 0.0050.015 0.45 0.31 0.028 0.003 18.12 0.11 0.25 — 0.008 0.10Sb, 0.0006B 14Steel 14 0.003 0.013 0.46 0.30 0.024 0.002 17.45 0.09 — 0.38 0.0130.11V, 0.82W, 0.06Zr, 0.0009Ca 15 Steel 15 0.007 0.010 0.44 0.29 0.0240.004 17.12 0.18 0.23 — 0.041 0.09Co, 0.0006Mg, 0.005REM 16 Steel 160.006 0.009 0.92 0.35 0.025 0.001 13.26 0.04 0.11 0.19 0.020 0.15Ni,0.29Cu, 0.11Ta 17 Steel 17 0.011 0.004 0.82 0.41 0.022 0.001 15.35 0.020.25 — 0.009 0.46Ni, 0.0004Ga 18 Steel 18 0.013 0.007 0.25 0.22 0.0280.002 16.89 0.09 — 0.46 0.041 — Comparative Ex. 1 Steel 19 0.011 0.0170.16 0.16 0.025 0.002 14.09 0.01 — 0.39 0.029 — 2 Steel 20 0.005 0.0080.15 0.36 0.025 0.003 10.47 0.09 — 0.29 0.026 — 3 Steel 21 0.006 0.0090.02 0.35 0.024 0.003 10.72 0.09 — 0.31 0.031 — 4 Steel 7 0.004 0.0080.20 0.09 0.025 0.002 13.87 0.03 0.13 0.12 0.036 — 5 Steel 7 0.004 0.0080.20 0.09 0.025 0.002 13.87 0.03 0.13 0.12 0.036 — 6 Steel 7 0.004 0.0080.20 0.09 0.025 0.002 13.87 0.03 0.13 0.12 0.036 — (Note) The underlinedshows values outside the scope of the invention.

TABLE 1-2 Finish annealing Cooling rate Grain Sn-concentrated layertemperature (° C./s) size Concentration Thickness Sn concentrationMaximum pit depth (° C.) 800 to 600° C. number degree (nm) (at %)* (μm)RN Inventive Ex. 1 Steel 1 860 15 6.5 2.1 6 0.12 299 7 2 Steel 2 970  77.5 2.8 8 0.30 322 6 3 Steel 3 870 15 6.0 2.3 5 0.07 313 8 4 Steel 4 930 8 7.5 3.0 10 0.64 288 6 5 Steel 5 860 15 6.5 2.1 10 0.43 356 6 6 Steel6 880 15 6.5 2.2 9 0.38 343 7 7 Steel 7 870  3 8.0 3.8 5 0.05 379 6 8Steel 8 930 10 6.5 2.7 4 0.06 275 9 9 Steel 9 930  4 7.0 3.8 2 0.05 2699 10 Steel 10 980 10 7.0 2.6 4 0.25 331 6 11 Steel 11 880 15 6.0 2.3 60.19 296 7 12 Steel 12 1000   4 8.0 3.5 5 0.23 157 8 13 Steel 13 860 156.5 2.3 4 0.11 265 7 14 Steel 14 960 10 7.5 3.0 6 0.12 245 6 15 Steel 15860 15 6.5 2.3 8 0.18 277 7 16 Steel 16 870 10 6.5 2.3 4 0.04 391 6 17Steel 17 880 10 6.5 2.3 3 0.02 386 6 18 Steel 18 990 15 7.0 2.3 5 0.09361 6 Comparative Ex. 1 Steel 19 950 10 6.5 2.3 2 0.01 645 5 2 Steel 20940 15 6.0 2.8 5 0.11 575 4 3 Steel 21 930 15 6.0 2.5 5 0.10 559 4 4Steel 7 1035  15 4.0 1.5 3 0.02 452 5 5 Steel 7 890 15 6.5 0.8 — 0.01532 5 6 Steel 7 890 30 6.5 0.8 — 0.01 525 5 *Minimum Sn concentration inSn-concentrated layer (Note) The underlined shows values outside thescope of the invention.

Test results are shown in Table 1-2. The grain size number shows themeasurements on the specimen cut out from the cold-rolled steel sheet.When the specimen subjected to the heat treatment was assessed in termsof the grain size number, the same results as those of the specimen ofthe cold-rolled steel sheet not subjected to the heat treatment wereobtained. It should be noted that, since the Sn-concentrated layer wasnot formed in Comparative Examples 5 and 6, the Sn concentration at andnear the border between the oxide film and the base material isdescribed in Comparative Examples 5 and 6.

As shown in Table 1-2, inventive Examples 1 to 18 show 400 μm or less ofthe maximum pit depth and 6 or more of RN and are thus excellent incorrosion resistance.

Comparative Example 1 whose Sn content does not satisfy the requirementsof the invention, Comparative Example 2 whose Cr content does notsatisfy the requirements of the invention, Comparative Example 3 whoseSi content does not satisfy the requirements of the invention,Comparative Example 5 whose heating condition does not satisfy theformula (I) and Comparative Example 6 whose cooling rate in thetemperature range from 800 to 600 degrees C. during the finish annealingstep exceeds 20 degrees C./s all show the maximum pit depth of more than500 μm and 5 or less of RN and thus are inferior in corrosionresistance. Though the Sn-concentrated layer is formed in ComparativeExample 4 whose grain size number is 4, the Sn concentration is notsufficient due to the influence of the grain size number. Consequently,though a certain degree of the pitting corrosion resistance is ensuredas shown by the 400 to 500 μm of the maximum pit depth, ComparativeExample 4 is inferior in rust resistance as shown by the RN of 5.

INDUSTRIAL APPLICABILITY

Ferritic stainless steel of the invention is suitable for exhaust systemcomponents of an automobile, a motor cycle, a commercial vehicle, aconstruction machine and the like that are subjected to heating in use.Examples of suitable exhaust system components include a converter case,a front pipe, a center pipe and a muffler.

1-13. (canceled)
 14. A ferritic stainless steel for exhaust systemcomponents excellent in corrosion resistance after heating, the ferriticstainless steel comprising: 0.015 mass % or less of C; 0.02 mass % orless of N; 0.03 mass % to 1.0 mass % of Si; 1.0 mass % or less of Mn;0.04 mass % or less of P; 0.01 mass % or less of S; 10.5 mass % to 22.5mass % of Cr; 0.02 mass % to 0.5 mass % of Sn; 0.003 mass % to 0.2 mass% of Al; one or both of 0.03 mass % to 0.35 mass % of Ti and 0.03 mass %to 0.6 mass % of Nb; and a remnant comprising Fe and inevitableimpurities, wherein a grain size number on a surface of the ferriticstainless steel is 6 or more, and 2 to 15 nm of a layer containing Sn ata concentration twice or more of a Sn concentration in a base materialis formed on the ferritic stainless steel when the ferritic stainlesssteel is heated at 673 K for 24 hours in the atmosphere.
 15. A ferriticstainless steel for exhaust system components excellent in corrosionresistance after heating, the ferritic stainless steel comprising: 0.015mass % or less of C; 0.02 mass % or less of N; 0.03 mass % to 1.0 mass %of Si; 1.0 mass % or less of Mn; 0.04 mass % or less of P; 0.01 mass %or less of S; 10.5 mass % to 22.5 mass % of Cr; 0.02 mass % to 0.5 mass% of Sn;
 0. 003 mass % to 0.2 mass % of Al; one or both of 0.03 mass %to 0.35 mass % of Ti and 0.03 mass % to 0.6 mass % of Nb; and a remnantcomprising Fe and inevitable impurities, wherein a grain size number ona surface of the ferritic stainless steel is 6 or more, and 2 to 15 nmof a layer containing Sn at a concentration twice or more of a Snconcentration in a base material is formed on the ferritic stainlesssteel.
 16. The ferritic stainless steel for exhaust system componentsexcellent in corrosion resistance after heating according to claim 14 or15, further comprising at least one of a first group and a second group,the first group consisting of one or more of 0.05 mass % to 1.5 mass %of Cu, 0.1 mass % to 1.2 mass % of Ni, 0.03 mass % to 3 mass % of Mo,0.03 mass % to 1 mass % of W, 0.05 mass % to 0.5 mass % of V and 0.01mass % to 0.5 mass % of Sb, the second group consisting of one or moreof 0.03 mass % to 0.5 mass % of Zr, 0.02 mass % to 0.2 mass % of Co,0.0002 mass % to 0.002 mass % of Ca, 0.0002 mass % to 0.002 mass % ofMg, 0.0002 mass % to 0.005 mass % of B, 0.001 mass % to 0.01 mass % ofREM, 0.0002 mass % to 0.01 mass % of Ga and 0.01 mass % to 0.5 mass % ofTa.
 17. The ferritic stainless steel for exhaust system componentsexcellent in corrosion resistance after heating according to claim 14 or15, wherein the Sn content is 0.02 mass % or more and less than 0.05mass % and/or 0.07 mass % or more and 0.3 mass % or less.
 18. Theferritic stainless steel for exhaust system components excellent incorrosion resistance after heating according to claim 16, wherein the Nicontent is 0.1 mass % or more and less than 0.5 mass %.
 19. An exhaustsystem component excellent in corrosion resistance after heating, theexhaust system component being made from a ferritic stainless steel, theferritic stainless steel comprising: 0.015 mass % or less of C;
 0. 02mass % or less of N; 0.03 mass % to 1.0 mass % of Si; 1.0 mass % or lessof Mn; 0.04 mass % or less of P; 0.01 mass % or less of S; 10.5 mass %to 22.5 mass % of Cr; 0.02 mass % to 0.5 mass % of Sn; 0.003 mass % to0.2 mass % of Al; one or both of 0.03 mass % to 0.35 mass % of Ti and0.03 mass % to 0.6 mass % of Nb; and a remnant comprising Fe andinevitable impurities, wherein a grain size number on a surface of theferritic stainless steel is 6 or more, and 2 to 15 nm of a layercontaining Sn at a concentration twice or more of a Sn concentration ina base material is formed on the ferritic stainless steel.
 20. Theexhaust system component excellent in corrosion resistance after heatingand being made from the ferritic stainless steel according to claim 19,wherein the ferritic stainless steel further comprises at least one of afirst group and a second group, the first group consisting of one ormore of 0.05 mass % to 1.5 mass % of Cu, 0.1 mass % to 1.2 mass % of Ni,0.03 mass % to 3 mass % of Mo, 0.03 mass % to 1 mass % of W, 0.05 mass %to 0.5 mass % of V and 0.01 mass % to 0.5 mass % of Sb, the second groupconsisting of one or more of 0.03 mass % to 0.5 mass % of Zr, 0.02 mass% to 0.2 mass % of Co, 0.0002 mass % to 0.002 mass % of Ca, 0.0002 mass% to 0.002 mass % of Mg, 0.0002 mass % to 0.005 mass % of B, 0.001 mass% to 0.01 mass % of REM, 0.0002 mass % to 0.01 mass % of Ga and 0.01mass % to 0.5 mass % of Ta.
 21. The exhaust system component excellentin corrosion resistance after heating and being made from the ferriticstainless steel according to claim 19 or 20, wherein the Sn content is0.02 mass % or more and less than 0.05 mass % and/or 0.07 mass % or moreand 0.3 mass % or less.
 22. The exhaust system component excellent incorrosion resistance after heating and being made from the ferriticstainless steel according to claim 20, wherein the Ni content is 0.1mass % or more and less than 0.5 mass %.
 23. A manufacturing method ofthe ferritic stainless steel for exhaust system components excellent incorrosion resistance after heating according to claim 14 or 15, whereinwhen the ferritic stainless steel is manufactured, a finish annealingtemperature in a cold rolling step is 1030 degrees C. or less, and whenthe ferritic stainless steel is cooled from a cold-rolled-sheetannealing temperature, a cooling rate in a temperature range from 800 to600 degrees C. is less than 20 degrees C./s.
 24. A manufacturing methodof the ferritic stainless steel for exhaust system components excellentin corrosion resistance after heating according to claim 14 or 15,wherein when the ferritic stainless steel is manufactured, a finishannealing temperature in a cold rolling step is 1030 degrees C. or less,and when the ferritic stainless steel is cooled from a cold-rolled-sheetannealing temperature, a cooling rate in a temperature range from 800 to600 degrees C. is less than 5 degrees C./s.
 25. A manufacturing methodof the exhaust system component excellent in corrosion resistance afterheating and being made from the ferritic stainless steel according toclaim 19 or 20, wherein, when the ferritic stainless steel forming theexhaust system component is manufactured, a finish annealing temperaturein a cold rolling step is 1030 degrees C. or less, and when the ferriticstainless steel is cooled from a cold-rolled-sheet annealingtemperature, a cooling rate in a temperature range from 800 to 600degrees C. is less than 20 degrees C./s.
 26. A manufacturing method ofthe exhaust system component excellent in corrosion resistance afterheating and being made from the ferritic stainless steel according toclaim 19 or 20, wherein when the ferritic stainless steel forming theexhaust system component is manufactured, a finish annealing temperaturein a cold rolling step is 1030 degrees C. or less, and when the ferriticstainless steel is cooled from a cold-rolled-sheet annealingtemperature, a cooling rate in a temperature range from 800 to 600degrees C. is less than 5 degrees C./s.
 27. The ferritic stainless steelfor exhaust system components excellent in corrosion resistance afterheating according to claim 16, wherein the Sn content is 0.02 mass % ormore and less than 0.05 mass % and/or 0.07 mass % or more and 0.3 mass %or less.
 28. The ferritic stainless steel for exhaust system componentsexcellent in corrosion resistance after heating according to claim 17,wherein the Ni content is 0.1 mass % or more and less than 0.5 mass %.29. The ferritic stainless steel for exhaust system components excellentin corrosion resistance after heating according to claim 27, wherein theNi content is 0.1 mass % or more and less than 0.5 mass %.
 30. Theexhaust system component excellent in corrosion resistance after heatingand being made from the ferritic stainless steel according to claim 21,wherein the Ni content is 0.1 mass % or more and less than 0.5 mass %.